All-optical networks (AON), using wavelength division multiplexing (WDM), have become attractive candidates for building wide area networks (WANs). Finding a logical topology and a routing scheme making optimum use of network resources is a challenging task. A complication is that the survivability of AON has become an important issue. In designing a fault tolerant WDM network, the primary lightpaths, the corresponding backup lightpaths, and the routing scheme have to be determined simultaneously in such a way that network resources are used in an optimum manner. In this paper we first develop an Integer Linear Formulation (ILP) for designing a fault-tolerant logical using shared path protection. The objective is to design a survivable logical topology and a routing over that topology in such a way that the overall congestion is minimized. This formulation can be solved to give us the optimum logical topology for small WDM networks. However, for larger networks, this approach becomes infeasible due the large number of constraints and integer variables. For such networks, we outline a simple heuristic algorithm to find a feasible logical topology.
Existing approaches for logical topology design and routing for multi-hop optical networks become intractable for large networks. One approach is to treat the logical topology design problem separately from the routing problem which can be solved as a LP problem. The straightforward formulation of the LP problem has been reported but this is also feasible only for relatively smaller networks since the basis size for the simplex method is O(n(superscript 3)) where n is the number of nodes in the network. In this paper, by exploiting the special structure of the routing problem, we present an efficient column generation scheme embedded into the revised simplex method. This approach makes it feasible to handle networks with relatively large number of nodes. To study the approach experimentally we have used a number of traffic based heuristics for generating the logical topologies. These include a variation of the well known HLDA heuristic and two simple traffic based heuristics to generate logical topologies based on regular graphs. Many researchers feel that regular graphs are not well suited for wide area optical networks. The interesting result is that logical topologies based on regular graphs perform quite well compared to others. This suggests that it is useful to consider regular graphs as possible topologies for wide area networks and should be included as potential candidates for large wide area networks.
In this paper we have examined the efficacy of virtual wavelength conversion in all-optical networks with respect to full wavelength conversion. It is well known that wavelength conversion reduces the blocking probability. All- optical full wavelength converters are not yet commercially available and wavelength conversion typically involves opto- electrical conversion. Virtual wavelength conversion avoids any change in the carrier frequency and is feasible using available technology. In this paper we have studied a dynamic wavelength assignment scheme using a distributed algorithm. We have shown that using virtual wavelength conversion, we can achieve the effect of full wavelength conversion without using wavelength converters in optical domain.
Fault management in WDM routed all-optical networks has mostly been addressed either by automatic protection switching or through loop-back recovery. These schemes are designed for managing single fault occurrence and generalization method to handle multiple faults are not known. Conventional routing schemes are static in nature (where the routers are programmed to realize the lightpaths between the end-nodes) and hence a fault management scheme needs to find a fault-free path between end-nodes using the settings of the routers. This paper considers the principle of survival route graphs to construct fault-free paths between end-nodes. As a result, the fault avoiding route between two end-nodes might be a multihop route in which the number of hops are limited to reduce the communication delay. The performance degradations of the network because of fault occurrence are studied through simulations and measured in terms of blocking probability and communication delay.
Fault-tolerant routing for WDM routed all-optical networks has not been studied in details in the literature except for single fault occurrence in a simple topology, like a ring. Conventional routing schemes, being static in nature, are difficult to be modified to handle faults as fault occurrences need dynamic rerouting. In this paper, we present a fault-tolerant routing scheme for WDM routed all- optical networks. This paper considers modifications to a dynamic routing scheme to handle rerouting in case of fault occurrence. The performance degradations of the network because of fault occurrence are then studied in terms of blocking probability and set-up time through simulations.
In this paper, we study an adaptive algorithm for routing in an all-optical wavelength routed network where a lightpath is dynamically created in response to a request for communication and the WDM channels constituting the lightpath are reclaimed when the communication is over. We have proposed three possible distributed strategies to determine, if possible, a lightpath when there is a request for communication. Each of these strategies have their advantages and disadvantages in terms of the expected blocking probability and the set-up time. We consider the Kautz digraph as the network topology and study the performance of our distributed strategies through simulation.
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